Nov 4 (Reuters) - GAM Holding AG has rebuffed an offer from Schroders Plc to sell its Systematic division, which handles the Swiss money manager’s Cantab quantitative hedge fund unit, the FT reported, citing two people familiar with the matter. GAM turned down the offer because it was concerned that selling one of its most attractive units separately would make it difficult to sell the whole company, the FT reported on Sunday.